The present invention relates to a semiconductor memory device, and particularly to an arrangement that can be effectively adapted to semiconductor memory devices of a highly integrated form.
Semiconductor memory devices have memory cell arrays in which a plurality of memory cells are arranged in the form of a matrix on a semiconductor chip. In the memory cell arrays are formed a plurality of word lines that extend in one direction. On the word lines are formed a plurality of data lines with an intermediate insulating film therebetween, the data lines extending in a direction at right angles with the word lines.
Accompanying the progress toward a high degree of integration, word lines and data lines have become finer. For instance, in a dynamic RAM (random access memory) of 256 K bits, the pattern width is about 2 .mu.m.
However, the yield decreases as the patterns become finer. Therefore, a so-called redundancy configuration has been employed to replace a failed or a malfunctioning bit, row or column with a spare element.
According to the study conducted by the inventors of the present invention, the portions that are replaced most frequently are word lines at the ends of the memory cell arrays.
The inventors attribute the cause to the following reasons.
The word lines are coated with an intermediate insulating film consisting of a phosphosilicate glass or the like, and data lines are formed thereon. Contact holes are formed in the intermediate insulating film prior to forming the data lines.
The thickness of a resist film for forming the contact holes is limited to, for example, about 1 .mu.m from the standpoint of precision for forming patterns of contact holes. Since the resist film is in a fluid state for such processing as spin coating and backing, the resist film thickness is less on protruded portions than in recessed portions. Therefore, the resist film is thinner on the word lines formed on the electrode layers of capacitors formed on a thick field oxide film than on other portions.
In particular, the resist film is thinnest at portions of word lines located at the ends of memory cell arrays. Since no word line exists on one side of these word lines, it is considered that the resist film tends to flow toward the direction where no word line exists. The thickness of the resist film varies depending upon the underlying pattern.
To form fine contact holes, dry etching is employed. Further, over-etching is performed so that the surface of the substrate is completely exposed.
The resist film is etched during dry etching by several thousands of angstroms overall. In the portions where the resist film is thin, in particular, the resist film is removed and the underlying intermediate insulating film is exposed. The intermediate insulating film which is exposed is easily etched. Therefore, the word lines under the intermediate insulating film are subjected to etching, causing defects such as line breakage.
Word line breakage develops locally in the portions where the resist film is particularly thin.
This defect develops not only in the word lines at the ends of the memory cell arrays but also in the dummy cell-selecting word lines in the dummy cell arrays.